There has been conventionally known a variable geometry turbocharger in which an opening area of a nozzle provided to eject exhaust gas into an exhaust turbine can be adjusted. In such a variable geometry turbocharger, at a low speed revolution zone of an engine having a small displacement, a gap between exhaust inlet walls forming a nozzle is reduced to reduce an opening area thereof, thereby increasing a flow speed of exhaust gas flowing into the exhaust turbine. Thus, the rotary energy of a turbine wheel can be increased to enhance supercharging performance of a charging compressor.
A sliding mechanism in which one of the exhaust inlet walls is slid toward the other exhaust inlet wall is employed for adjusting the gap between the exhaust inlet walls, and the slide mechanism is driven by a pneumatic actuator (for example, Patent Document 1). Here, the pneumatic actuator is typically formed by a cylinder and a piston that slides within the cylinder, and the piston is slid in one direction by air pressure of a compressed air and in the other direction by a coil spring biasing the piston theretoward. When the piston is slid in the other direction, supply of the air pressure is interrupted.
While Patent Document 1 discloses the piston reciprocated by the air pressure and spring force, a hydraulic drive device may be employed in place of the pneumatic actuator to control the opening degree of the nozzle (Patent Document 2). In other words, a slide mechanism that varies the opening area of the nozzle is driven by the hydraulic drive device. In such a hydraulic drive device, a hydraulic servo piston is reciprocated by switching oil supply for hydraulic chambers provided on both sides of the hydraulic servo piston to transfer this reciprocating motion to the slide mechanism, thereby controlling the opening degree.    Patent Document 1: JP-A-11-72008    Patent Document 2: JP-T-2003-527522